A simulation study of the impacts of population subdivision on the mountain brushtail possum Trichosurus caninus Ogilby (Phalangeridae: Marsupialia) in south-eastern Australia. I. Demographic stability and population persistence

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Forest fragmentation and the associated problem of population subdivision is a major factor threatening the long-term persistence of many forest-dependent taxa. VORTEX, a computer program for population viability analysis (PVA), was used to simulate the impacts of fragmentation and subdivision on the demography of populations of the mountain brushtail possum Trichosurus caninus Ogilby, which is a forest-dependent species of arboreal marsupial that inhabits wet sclerophyll forests and rainforests in eastern Australia. In this study, hypothetical populations of 100, 200 and 400 animals were partitioned into one to 10 subpopulations that were, in turn, linked by varying rates of inter-patch migration. These levels of population subdivision may be typical in extensive areas of wood production montane ash forest in central Victoria where there are limited and generally very small patches of old growth forest (< 10 ha) that will become increasingly important refugia for many species of hollow-dependent fauna such as T. caninus. The results of the analysis demonstrated that the impacts of population subdivision on population demography were usually negative. In almost all of the scenarios, a single population was more stable than an ensemble of subpopulations of comparable initial size, irrespective of the rate of inter-patch migration. There were marked differences in the behaviour of subpopulations initially comprised of 10 and 20 T. caninus and those comprised of approximately 40 or more animals. Small subpopulations were predicted to exhibit high inherent levels of population instability, probably as a result of factors such as demographic stochasticity and inbreeding depression, and were characterised by high rates of extinction, small or negative values for subpopulation and metapopulation growth, and large fluctuations in population size. The effects of the dispersal or migration of animals between subpopulations on their persistence were complex and related to the final size of the populations that resulted from the process of subdivision. Amongst ensembles of subpopulations which initially comprised 40 or more animals, increasing the rate of dispersal had a range of positive effects on subpopulation and metapopulation dynamics. These included: higher rates of, and smaller fluctuations in, subpopulation and metapopulation growth, and, lower probabilities of subpopulation and metapopulation extinction. Many of these trends were reversed when the metapopulation structure was comprised initially of small subpopulations of 10–20 T. caninus. In such cases, accelerated rates of migration were predicted to have a detrimental impact on most measures of population demography including the rate of extinction. The results of the study demonstrated the potential values of PVA, both in examining the role of dispersal in metapopulation dynamics and studying changes in population demography and persistence associated with population subdivision.